For any melt processible thermoplastic polymer composition, there exists a critical shear rate above which the surface of the extrudate becomes rough and below which the extrudate will be smooth. See, for example, R. F. Westover, Melt Extrusion, Encyclopedia of Polymer Science and Technology, Vol. 8, pp 573-81 (John Wiley & Sons 1968). The desire for a smooth extrudate surface competes, and must be optimized with respect to, the economic advantages of extruding a polymer composition at the fastest possible speed (i.e. at high shear rates).
Some of the various types of extrudate roughness and distortion observed in high and low density polyethylenes are described by A. Rudin, et al., in Fluorocarbon Elastomer Aids Polyolefin Extrusion, Plastics Engineering, March 1986, on 63-66. The authors state that for a given set of processing conditions and die geometry, a critical shear stress exists above which polyolefins such as linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene suffer melt defects. At low shear rates, defects may take the form of “sharkskin”, a loss of surface gloss that in more serious manifestations appears as ridges running more or less transverse to the extrusion direction.
At higher rates, the extrudate can undergo “continuous melt fracture” becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, LLDPE and HDPE can also suffer from “cyclic melt fracture”, in which the extrudate surface varies from smooth to rough. The authors state further that lowering the shear stress by adjusting the processing conditions or changing the die configuration can avoid these defects to a limited extent, but not without creating an entirely new set of problems.
For example, extrusion at a higher temperature can result in weaker bubble walls in tubular film extrusion, and a wider die gap can affect film orientation.
There are other problems often encountered during the extrusion of thermoplastic polymers. They include a build-up of the polymer at the orifice of the die (known as die build up or die drool), increase in back pressure during extrusion runs, and excessive degradation or low melt strength of the polymer due to high extrusion temperatures. These problems slow the extrusion process either because the process must be stopped to clean the equipment or because the process must be run at a lower speed.
Certain branched processing aids are known to partially alleviate melt defects in extrudable thermoplastic hydrocarbon polymers and allow for faster, more efficient extrusion.
U.S. Pat. No. 7,375,157 (Amos et al.) describes the use of a fluoropolymer having long chain branching for use as a polymer melt additive. The fluoropolymers are derived from bisolefins or halogenated olefins, which comprise a halogen that is readily abstracted during the polymerization, such as bromine or iodine.
U.S. Pat. Publ. No. 2010/0311906 (Lavallee et al.) also describes the use of a fluoropolymer having long chain branching for use as a polymer melt additive. The fluoropolymers are derived from a fluorinated olefin monomer and a fluorinated bisolefinic ether.